1. Field of the Invention
The invention relates to paper machine clothing and more particularly relates to fabrics useful as dryer felts, belts made therefrom and their use on papermaking machines.
2. Brief Description of the Prior Art
The art is replete with descriptions of dryer felts and dryer felt fabrics. In spite of the wide variety of materials available, the ideal dryer farbic is yet to be found for use in fabricating papermachine dryer belts.
One of the problems associated with the prior art dryer felt fabrics is an uneven moisture profile in the web of paper carried by the dryer felt through the drying section of a papermaker's machine. The problem is fully described in U.S. Pat. No. 3,867,766. In brief, instead of having the same moisture content across the width of the paper web, a higher moisture content develops in the center of the paper web being dried and lower moisture contents develop along the lateral edges of the moving paper web. This is undesirable in that it necessitates overdrying of the web which affects the quality of the final paper product and consumes large amounts of additional energy. Furthermore, the cost of paper produced in this manner is higher because the lower final moisture in the product means a correspondingly higher proportion of fiber. Finally, it is known that paper webs with higher moisture contents have various properties making them more desirable in secondary operations. The solution to this problem offered by the patentee in the U.S. Pat. No. 3,867,766 is to provide a dryer belt having greater permeability in the center than along the lateral edges; i.e.; along the felt edges in the machine direction. The difference in permeability is achieved by varying the diameter of the machine direction yarns of the dryer felt fabric in the different zones; i.e.; at the center and along the lateral edges. It will be appreciated that such an approach complicates the weaving of the fabric and increases costs.
A different approach to the above-described problem was taken by the patentee of U.S. Pat. No. 3,151,953. Variable pressures are exerted on the dryer felt across its width, i.e; higher pressures at the center so that the paper web is pressed to a higher degree in the center against the heated drying surface. An acceleration of drying rate at the center of the paper web is achieved in comparison to along the lateral edges of the moving paper web. It will be appreciated that adjustment of the variable pressures across the width of the machine is difficult and not subject to objective controls. Uniformity in the final paper product is difficult to achieve.
Other approaches to obtaining uniform moisture profile in the drying paper web have included varying the reeding of the dryer felt fabric. Variable reeding of the fabric is undesirable for many reasons, including the fact that the fabric must then be woven to width (putting severe limitations on the use of stock rolls). Also, the fabric tension is uneven across this type of fabric, leading to fabric instability.
Treatment of fabric lateral edges parallel to the machine direction to reduce permeability is not desirable. Such treatments generally compromise guidability of the dryer felt.
By the present invention, a uniform moisture profile in paper webs passing through the dryer section of a papermakers' machine is achieved by the employment of a dryer felt having a surface of controlled paper contacting area. The dryer felts of the invention are stable, exhibit excellent guidability and are readily operable with a minimum of adjustments and operating difficulties. The dryer felt fabrics from which the felts are made need not be custom woven to width but may be cut from stock weavings. Drying rates are controlled by controlling the surface area contact between the paper web and the dryer felt rather than by controlling air flows through the dryer felt, the latter control being imprecise and difficult to maintain.
In recent years, dryer fabrics have been developed which are constructed partially or entirely from monofilaments. However, such fabrics have not been entirely satisfactory when employed in fashioning dryer felts. The monofilaments are subject to abrasion and hydrolysis. In some prior art constructions the load bearing machine direction monofilament yarns may be rapidly degraded under some conditions of use so that the life of the dryer felt is shortened.
In the preferred embodiment structured fabrics of the present invention, employed as dryer felts, the cross-machine direction yarns are predominant on both fabric surfaces. machine direction yarns are thus protected from direct contact with degradative elements. The overall operating life of the dryer felt is significantly increased over felts where the machine direction yarns are in contact with the hot cans on the sheet side or the felt carrying rolls on the back side.
Other U.S. patents bearing descriptions representative of the state of the art in regard to dryer fabrics are U.S. Pat. Nos. 3,573,164 and 3,905,863.